Method and apparatus for manufacturing spiral pipe

Abstract

In the manufacturing of a spiral pipe, there is first determined a maximum curvature which should be imparted to a strip of pipe forming material in order that a desired residual moment will be imparted to the finished pipe after the strip bent to the maximum curvature is allowed to spring back from the bent condition, which maximum curvature is determined based on the thickness, Young's modulus and yield stress of the strip, the desired curvature of the finished pipe, and the residual moment to be imparted to the finished pipe. By adjusting the position of three rows of forming rolls positioned at the apexes of a triangle, the flat strip is continuously bent into a spiral that has the thus determined maximum curvature. Then, the spirally formed strip is allowed to spring back toward its unbent shape to the diameter of the finished pipe, being stopped in the expansion due to the springback if it reaches the finished pipe diameter before completing springback or being expanded if it completes springback before it reaches the finished pipe diameter. Finally, the seam of the formed strip is welded when the strip is at the desired diameter of the finished pipe.

What is claimed is: 
     
       1. In a method of manufacturing spiral pipe using a forming apparatus having three forming roll means at the apexes of a triangle and positioned along the circumference of the pipe being manufactured and in which method a first strip of pipe material is fed in the longitudinal direction thereof through said three roll means and is continuously bent into spiral form by pressing one of the forming roll means which is inside the circumference against the inner surface of the strip toward the outside of the pipe and pressing the other two forming roll means which are respectively ahead of and behind said inside forming roll means in the direction of movement of the strip against the outer surface of the strip toward the inside of the pipe, and the seam of a spirally formed strip is continuously welded, the improvement which comprises the steps of: determining, based on the thickness of the strip and the yield stress of the material of the strip and the curvature of the finished pipe, a maximum curvature 1/ρ i  of the strip before spring back expressed as ##EQU11##  where σ y  =yield stress of the material strip   E=Young's modulus of the material of the strip   2t=thickness of the material strip   ρ p  =radius of the finished pipe and γ is derived from ##EQU12##  where M p  is the desired residual moment, and which maximum curvature has a smaller radius of curvature than the radius of curvature of the finished pipe and which is sufficiently great, when it is desired that the finished pipe have a positive residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is larger than the radius of curvature of the finished pipe, and when it is desired that the finished pipe have a zero residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a curvature which is equal to the curvature of the finished pipe, and when it is desired that the finished pipe have a negative residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is smaller than the radius of curvature of the finished pipe;   continuously forming the flat strap into a spiral having said maximum curvature by adjusting the positions of said forming roll means in a direction transverse to the thickness of the strip; and, prior to the welding step,   when it is desired to impart a positive residual moment to the finished pipe, exerting a force on the exterior of the strip subsequent to the forming roll means in a direction of movement of the strip for restraining the springback of the strip to a curvature corresponding to the curvature of the finished pipe, and when it is desired to impart a negative residual moment to the finished pipe, allowing the pipe to spring back to the unstressed bent form and then exerting a force on the strip from the interior of the strip subsequent to the forming roll means in the direction of movement of the strip for expanding the strip to a curvature corresponding to the curvature of the finished pipe.   
     
     
       2. In a method of manufacturing spiral pipe using a forming apparatus having three forming roll means at the apexes of a triangle and positioned along the circumference of the pipe being manufactured and in which method a flat strip of pipe material is fed in the longitudinal direction thereof through said three roll means and is continuously bent into spiral form by pressing one of the forming roll means which is inside the circumference against the inner surface of the strip toward the outside of the pipe and pressing the other two forming roll means which are respectively ahead of and behind said inside forming roll means in the direction of movement of the strip against the outer surface of the strip toward the inside of the pipe, and the seam of the spirally formed strip is continuously welded, the improvement which comprises the steps of: measuring the thickness of the strip, and measuring the yield stress by providing a roller leveler ahead of the forming roll means, measuring the leveling load applied by said roller leveler, and converting the measured load into yield stress;   determining, based on the thickness of the strip and the yield stress of the material of the strip and the curvature of the finished pipe, a maximum curvature 1/ρ i  of the strip before spring back expressed as ##EQU13##  where σ y  =yield stress of the material strip   E=Young's modulus of the material of the strip   2t=thickness of the material strip   ρ p  =radius of the finished pipe and γ is derived from ##EQU14##  where M p  is the desired residual moment, and which maximum curvature has a smaller radius of curvature than the radius of curvature of the finished pipe and which is sufficiently great, when it is desired that the finished pipe have a positive residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is larger than the radius of curvature of the finished pipe, and when it is desired that the finished pipe have a zero residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a curvature which is equal to the curvature of the finished pipe, and when it is desired that the finished pipe have a negative residual moment, to cause the pipe, when the force causing such bending is released, to spring back to an unstressed bent form with a radius of curvature which is smaller than the radius of curvature of the finished pipe; continuously forming the flat strip into a spiral having said maximum curvature by adjusting the positions of said forming roll means in a direction transverse to the thickness of the strip; and, prior to the welding step,   when it is desired to impart a positive residual moment to the finished pipe, exerting a force on the exterior of the strip subsequent to the forming roll means in a direction of movement of the strip for restraining the springback of the strip to a curvature corresponding to the curvature of the finished pipe, and when it is desired to impart a negative residual moment to the finished pipe, allowing the pipe to spring back to the unstressed bent form and then exerting a force on the strip from the interior of the strip subsequent to the forming roll means in the direction of movement of the strip for expanding the strip to a curvature corresponding to the curvature of the finished pipe.   
     
     
       3. The improvement as claimed in claim 1, in which said adjusting of the position of the forming roll means comprises moving only the forming roll means that is positioned ahead of, in the direction of movement of the strip, the internal forming roll means while keeping the internal forming roll means and the forming roll means that is positioned behind the internal forming roll means in predetermined positions. 
     
     
       4. The improvement as claimed in claim 1, in which the yield stress and the thickness of the strip being formed is measured, and the position of the forming rolls is adjusted based on the values thus determined. 
     
     
       5. The improvement as claimed in claim 1, in which one of the characteristics of the strip being formed taken from the characteristics of yield stress and thickness is measured. 
     
     
       6. The improvement as claimed in claim 1, further comprising measuring the bending load applied to the strip by the forming roll means and the position of the forming roll means relative to the path of movement of the strip during the forming step, and adjusting the position of the forming roll means for making the ratio of a change in the position of the forming roll means to a change in the bending load such that the desired residual moment is obtained. 
     
     
       7. A spiral pipe manufacturing apparatus which comprises: a forming apparatus having three forming roll means disposed at the apexes of a triangle and positioned along the circumference of the pipe being manufactured for continuously bending a flat strip into spiral form, one of said roll means being an internal forming roll for contacting the inner surface of the strip and pressing toward the outside of the pipe and the other two of said forming roll means being outer rolls and respectively being ahead of and behind said internal forming roll in the direction of movement of the strip and for contacting the outer surface of the strip and pressing it toward the inside of the pipe, a welding apparatus provided behind said forming apparatus in the direction of movement of the strip and along the circumference of the pipe being manufactured for continuously welding the seam of the spirally formed strip, a plurality of holding rolls positioned along the path of movement of the strip behind said forming apparatus for engaging one side of the strip for holding the strip in a shape equal to the desired diameter of the pipe, and a bending moment imparting roll means adjacent to and behind the forming apparatus in the direction of movement of the strip and ahead of the first of said holding rolls and on the opposite side of the strip from said holding rolls and adjustably movable in the direction of the pipe diameter for engaging the strip for holding it in a position as it moves past said bending moment imparting roll means for making the diameter of the spirally bent strip equal to the desired diameter of the finished pipe. 
     
     
       8. An apparatus as claimed in claim 7, in which said other two forming roll means are adjustably movable toward said one of said forming roll means, and further comprising means for sensing the characteristics of the strip being bent and adjusting the position of at least one of the other two forming roll means in response to the sensed characteristics for applying the desired bending force to the strip. 
     
     
       9. An apparatus as claimed in claim 8, in which said sensing and adjusting means comprises means for adjusting the position of only one of said other two forming roll means. 
     
     
       10. An apparatus as claimed in claim 7, in which said plurality of holding rolls are on the inside of the strip, and said bending moment imparting roll means comprises an outer bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the outside of the circumference of the pipe being manufactured. 
     
     
       11. An apparatus as claimed in claim 8, in which said plurality of holding rolls are on the outside of said strip, and said bending moment imparting roll means comprises an inner bending moment imparting roll means mounted on said apparatus for movement toward and away from the path of movement of the strip and on the inside of the circumference of the pipe being manufactured.